LED light source device comprised of a plurality of LED (Light Emitting Diode) strings is rapidly propagated for a wider use in lighting devices and backlight assemblys for LCD panel.
Generally, an LED having a high brightness may be used for various application devices including backlight assemblies for LCD, monitors and televisions (hereinafter collectively referred to as “monitor”). The LEDs applied in a large-sized monitor are generally implemented in one or more strings connected in series.
In order to apply a backlight assembly to the LCD monitor, one of two basic technologies are employed. A first technology is to use one or more strings comprised of a white LED, where the white LED generally includes a blue LED having a fluorescent material. The fluorescent material absorbs the blue light generated by the LED to emit a white light. A second technology is to have one or more individual strings comprised of a colored LED in adjacent arrangement, whereby combined colors come to look white.
However, due to characteristic (e.g., forward voltage drop) deviation among LED elements comprising the LED strings, even LED strings comprising same types of LEDs show mutually different electrical features (e.g., voltage drop). Because of that, in order for the same current to flow through each LED string, there is a need to add a constant current control block connected in series to each LED string for compensating different voltage drops, which is applied with a dissipative active element for compensating the different voltage drops of the LED strings.
However, the dissipative active element suffers from disadvantages in that the dissipative active element, being a significant heat source, increases heat-radiating cost to an entire LED driver, and requires a large capacity of power supply device due to reduced power transmission efficiency.
For example, as illustrated in FIG. 1, an LED driving circuit according to the conventional art includes a power supply 10, an LED string 20 and a constant current control module 40.
In order to operate the LED string 20 in an equal brightness, the constant current control module 40 receives a current passing through the LED string 20 and the constant current control module 40, changes an output amplitude and maintains a constant current.
Furthermore, the constant current control module 40 generates a PWM (Pulse Width Modulation) control signal for linearly controlling the brightness dimming whereby duty ratio can be controlled. The constant current control module 40 functions as a load absorbing a voltage difference caused by a voltage increase/decrease of the LED string 20 by performing a linearly switching operation.
The elements comprising the constant current control module 40 receives a requirement of high element ratings including high voltage and high power, if the number of serially connected LED elements comprising the LED string 20 is increased to thereby supply a high power. Furthermore, an increased driving power deepens a heat-generating problem resultant from a high power of the linearly operating switch elements. As a result, a range of selecting elements for forming the illustrated constant current control module 40 is narrowed to increase the manufacturing cost.